Transistors, such as MOSFETs (metal oxide semiconductor field effect transistors) or IGBTs (insulated gate bipolar transistors), are widely used as electronic switches for switching electronic loads. For this, a load path of the transistor is connected in series with the load between terminals for a supply voltage, where the load is switched on when the transistor is in an on-state (switched on) and is switched off when the transistor is in an off-state (switched off).
When the load is an inductive load, such as, e.g., a motor, a magnetic valve, or a choke, the load stores magnetic energy when the switching element is in its on-state. This energy may result in a high voltage across the load path of the transistor after switching off, unless measures are taken that allow this energy to dissipate. According to a first approach, a freewheeling circuit may be connected in parallel with the load that allows a current through the load to continue to flow until the energy is dissipated.
According to a second approach, the transistor itself is used to dissipate the energy. In this case, the transistor is operated in a high-ohmic state in which an on-resistance of the transistor is higher than in a normal operation state in order to dissipate the energy. Operating the transistor in the high-ohmic state may include a control process that controls a drive signal applied to the control terminal of the transistor such that a voltage between one of the load terminals and the control terminal of the transistor is limited to a given voltage value.
In the second approach, when the energy is dissipated in the transistor, the current through the transistor decreases and, according to the characteristic curve(s) of the transistor, the amplitude of the drive signal decreases. However, there are transistors in which instabilities may occur when these transistors are operated at low drive signals and/or at low load currents.
There is, therefore, a need to provide an electronic circuit with a transistor that is suitable to drive an inductive load and in which instabilities can be avoided, and to provide a method for a stable operation of a transistor.